The reason cannot is a preferred translation is because of the grammatical structure of the Japanese sentence. いけません comes from the verb 行く (which if you remember, means to go). It's conjugated in the negative potential form. The potential form being いける (to be able to go), and the negative potential form being いけない (to be unable to go).
The word いけない most frequently has the connotation of something being prohibited. It's a little like saying thats a "no go" in English. Sometimes, when translating into English, it may feel more natural to translate this type of command structure as "don't do..." but "you can't do..." is also a valid, if not more accurate, translation.

してはダメです also works as "You cannot", but just to flesh it out a little, いけません is a way of saying, "Do not", as it would be written on a sign. This is pretty much synonymous with, "You cannot", so both should be right really. Alternatively, if you want to say, "Don't use a lot of money", as someone recommending an action to another, it can be put thusly: お金をたくさん使わないようにしてください。Hope that helps!

To those saying “cannot” is incorrect and should be “may not”......I disagree. If that ever was the case, I think it’s outdated by now. Hardly anyone I know says “you may not do this”. You sound like a square when you say it like that. Everybody says can’t. “Can I add 4 cups of paprika?” “No dude you can’t add 4 cups of paprika! That’ll taste like ❤❤❤❤!” So much more natural that way.

"....te wa ikemasen" can have various translations, including "cannot," none of them are literal. As "cannot" it does not indicate physical impossibility. It means that there is some reason the thing is not to be done.

I'm a native English speaker and it's unclear to me what "You cannot use a lot of money." means. I understand its literal meaning, but the literal meaning is so unusual, it seems much more likely that anyone uttering the sentence must have misspoken.

Is the speaker trying to give financial advice to the listener? Is the speaker a banker informing the listener of the funds in their account? Is the speaker a merchant explaining they don't access cash payments? Is the listener on some sort of game show where weird arbitrary rules like "can't use large amounts of money" are in placed to make the game being played interesting?

It says what it says and context determines how that is to be interpreter. Any of the contexts you mention are possible. If more specific language is needed, Japanese can supply it but, typically, all that will be said is what is needed in context.

Some sentences, like "The sky is blue," make immediate sense to everyone because they reference common experience and do not depend on a contextual situation. Other sentences, like "You may not use a lot of money," beg for context to be fully understood. Duo, and other language courses, give us sentences of this second type without giving us context. As long as the sentence has the necessary elements to make it at grammatical sentence, the best approach is take it literally, imagine a context if possible, and deal with it as it stands. The point is to learn the vocabulary and structure, not to worry about whether or in what context the sentence might be uttered. It is sufficient that it is a possible grammatically complete sentence.

While I admire the general sentiment of your advice, pragmatically I don't know that I can endorse it.

Let's say you're learning a new language on Duolingo, and the "literal" translation provided to you for some foreign sentence is "The sky cries blue". How do you use this translation to help you learn the meaning of the original words in the foreign language?

Is it saying that it's raining? But then what's the relevance of "blue"? Is it saying that it's raining, but the sky is also clear? Is it saying that the raindrops themselves are blue? Or is "crying blue" an idiom for snow? hail? lightning bolts? something else?

Without knowing the answers to these questions, you have not learned vocabulary. You do not understand the sentence, and thus cannot apply your "knowledge" in a new situation. You wouldn't know when to say it, because you don't know what it means. When someone says it to you, you don't know what they mean.

If you know that it seems to be nonsense, you already know the standard glosses for the words.

If Duo hasn't given you a meaning, Duo isn't going to explain it for you. (Duo usually works sentences both ways; so, you can see what Duo thinks it means in English.) If a dictionary doesn't help and the sentence still seems like nonsense, it probably is.
There is a famous sentence in linguistics, "Green ideas sleep furiously," to demonstrate that grammatically complete structures can be formed from words that logically cannot be combined. Grammar doesn't guarantee logic. But I haven't seen such a sentence in this course at all.

What we mostly see, that cause trouble, are sentences that can only apply to a context known to the parties in a conversation. The sentences are clear enough in themselves (like this one about using money) but you can't make complete sense of them without supplying context. The fact is that the who, where, when, what and why of the supplied context is not going to change the information given in the sentence. The sentence can stand as given in any context to which it can be applied and the best translation can vary accordingly. Just figure out what is actually said, imagine a context (or contexts) in which it might be said, and move on.

Japanese is a highly contextual language that tends to leave out what can be presumed to be understood. What is said may fit multiple contexts but literally mean what it says in all of them.

While I see course contributors in other courses actively look at the comments here, I don't think it is the case for Japanese, and I have not seen for a year that a single feedback from the whole Japanese sentence forum is ever taken by these volunteers.